PTC-elements can be built up of ceramics or organic polymers for example. Organic polymers are doped with conductive particles, like graphite or carbon black for example. The conductive particles allow electrical current flow under low temperatures as they are in contact which each other, but through heating the polymer matrix expands and consequently the particles loose their contact and finally an electrical current flow is hindered or even not longer possible.
EP 0 731 475 describes a positive temperature coefficient composition comprising, by weight, based on total composition, 10-30% electrical conductive phase, 10-40% chlorinated, maleic anhydride grafted, polypropylene resin, and 30-80% organic medium capable of solubilizing the resin. The composition contains electrical conductive fillers such as carbon black, graphite and the like. It further describes a sheet comprising a cast layer of the positive temperature coefficient composition which has been heated to remove volatile organic solvent.
EP 0 932 166 A1 describes a polymeric PTC composition comprising an organic polymer and conductive particles having a melting point of not less than 2000° C. dispersed therein, with the conductive particles being treated with a coupling agent. The conductive particles are selected from the group consisting of a metal, a metal carbide, a metal boride, a metal silicide and a metal nitride and are contained in an amount of 50 to 99% by weight based on the composition.
U.S. Pat. No. 6,479,575 describes a conductive polymer blend composition exhibiting PTC behavior, comprising a first polymer having a melt flow index of 5.0 to 15.0 and constituting approximately 5 to 45 percent by weight of said composition, a second polymer having a melt flow index less than 1.0 and constituting approximately 2 to 40 percent by weight of said composition, and at least one conductive particulate component. Preferably a mixture of at least two different conductive particulates is used in order to minimize the resistance hysteresis effect.
US 2002/0093007A1 describes an organic positive temperature coefficient thermistor comprising a matrix of at least two polymers, a low-molecular weight organic compound, and conductive particles having spiky protuberances, wherein said matrix contains a thermoplastic elastomer. The amount of the conductive particles is relatively high and should be 1.5 to 8 times as large as the total weight of the polymer matrix and low-molecular organic compound (the total weight of organic components inclusive of curing agent and other additives).
US 2003/0218530 A1 describes an organic positive temperature coefficient thermistor comprising a thermistor body comprising a high-molecular weight organic compound-containing matrix and metal particles, wherein a non-metallic powder of conductive non-metallic fines attaches to surfaces of the metal particles. The use of epoxy resins is not mentioned in that document.
The effect of coupling agents on thermal and electrical properties of mica/epoxy composites is described in the Journal of Applied Polymer Science, Vol. 56, 1339-1347 (1995), in particular the influence of 3-aminopropyltriethoxysilane, 3-glycidyloxypropyltrimethoxysilane, and neoalkoxytri(dioctylpyrophosphato)zirconate on thermal expansion behavior, dielectric strength, and arc resistance of mica/epoxy composites is described there.
A general overview about temperature-sensitive PTC conductive polymer composite is given in the Encyclopedia of Polymer Science and Technology: Conductive Polymer Composite, Wiley, 2005.
Fournier describes in the Journal of Materials Science Letters 16 (1997) 1677-1679 the positive temperature coefficient effect in carbon black/epoxy polymer composites.
US 2006/0118767 A1 describes an anisotropic conductive adhesive having PTC characteristics comprising an insulating adhesive component and a plurality of conductive particles dispersed in the insulating adhesive component, wherein the insulating adhesive component contains a crystalline polymer. The crystalline polymer is selected from copolymers of a monomer having ester group, ether group, methylene group or polar group. Examples are polyamide resins, polymethylmethacrylates and polyvinylbutyral resins. The crystalline polymer provides PTC characteristics only when it is added in an amount of 30 wt % to 70 wt % relative to the insulating adhesive component. Additionally, the adhesive may comprise thermosetting resins such as epoxy resins or radical polymerizable compounds. By using an additional thermoplastic resin, the anisotropic conductive adhesive can be prepared in the form of a film.
US 20100038025 A1 describes a planar structure comprising at least one layer of an adhesive within which heat can be generated, wherein the adhesive is a hotmelt adhesive and a posistor. Preferably, the hotmelt adhesive is consisting of polyolefins, copolymers of polyolefins, ionomers, polyamides and/or copolymers of polyamides. The electrically conductive materials, which are preferably used are carbon nanotubes and/or carbon nanofibers. The use of graphite and/or carbon black is also possible.